Field of Invention
The present invention relates generally to vehicle navigation systems. More particularly, the present invention relates to an improved dead reckoning module for use in a vehicular navigation system.
Current vehicular navigation systems are hybrids which utilize one or more of the following position determining means to locate a vehicle. The positioning determining means include: global positioning system satellites (GPS), dead reckoning systems, and map databases. Typically, one among these systems will serve the primary navigation system function while the remaining determining means are utilized to re-calibrate cumulative errors in the primary system. Each determining means has its advantages and limitations.
GPS is an electromagnetic wave positioning system utilized to determine a vehicle's position. GPS includes Navstar GPS and its successors, i.e., differential GPS (DGPS), WAAS, or any other electromagnetic wave positioning system. Navstar is a GPS system which uses space based satellite radio navigation developed by the U.S. Department of Defense. GPS receivers provide users with continuous three-dimensional position, velocity, and time data when they have unobstructed view of at least four satellites. Navstar GPS consists of three major segments: space, control, and end-user segments. The space segment consists of a constellation of twenty-four operational satellites placed in six orbital planes above the Earth's surface. The satellites are in circular orbits and in such an orientation as to normally provide a GPS user with a minimum of five satellites in view from any point on earth at any one time. The satellite broadcasts a RF signal, which is modulated by a precise ranging signal and a coarse acquisition code ranging signal to provide navigation data. This navigation data, which is computed and controlled by the GPS control segment for all GPS satellites, includes the satellite's time, clock correction and ephemeris parameters, almanac and health status. The user segment is a collection of GPS receivers and their support equipment, such as antennas and processors which allow users to receive the code and process information necessary to obtain position velocity and timing measurements. There are two primary disadvantages to GPS positioning as it pertains to vehicular navigation. First, errors are imposed on the portion of the GPS signals accessible to civilians. The government imposes errors in the range of 100 meters. In urban environments, this can result in inadequate navigation capabilities due to the close proximity of streets, some of which are spaced apart by less than 100 meters.
The second disadvantage of GPS is that when the user is in urban environments with many scattering objects, such as buildings, it may not be possible to receive information from enough satellites to make an adequate position determination. For this reason, GPS is typically utilized in a hybrid navigation system with other position determining means such as dead reckoning and map databases.
Prior systems use a road network stored in a map database to calculate current vehicle positions. These systems send distance and heading information, derived from either GPS or dead reckoning, to perform map matching. Map matching calculates the current position based on the road network stored in the database and the input position and heading data. These systems also use map matching to calibrate sensors. The map matching, however, has inherent inaccuracies because map matching must look back in time and match data to a location. As such, map matching can only calibrate the sensors or serve as a position determining means when an absolute position is identified on the map. However, on a long straight stretch of highway, sensor calibration or position determination using map matching may not occur for a significant period of time.
Current land-based dead reckoning systems use vehicle speed sensors, rate gyros, reverse gear hookups, and wheel sensors to "dead reckon" the vehicle position from a previously known position. This method of dead reckoning is susceptible to sensor error and to cumulative error. Furthermore, systems that use odometers and reverse gear hookups lack portability due to the required connections. Moreover, the systems are hard to install in different cars due to differing odometer's configurations. Additionally, each odometer may generate a different number of pulse counts in the transmission. Odometer data also varies with temperature, load, weight, tire pressure and speed. Furthermore, connections to cruise control or ABS sensors may bring up safety concerns.
Accordingly, there is a need for a portable vehicular navigation system which is flexible, accurate, efficient and cost-effective in determining current position from a previous position.